1. Technical Field
This invention relates generally to cryogenics, and more particularly to a superconducting magnet made from materials such as the copper-oxide superconducting materials that become superconductive at relatively high temperatures.
2. Background Information
Some known materials exhibit superconductivity when cooled with liquid helium to four degrees above absolute zero (four kelvins). Others, called high temperature superconductor (HTS) materials, become superconducting at higher temperatures and so they can be cooled with liquid nitrogen. Since liquid nitrogen is relatively abundant and inexpensive, such HTS materials promise substantial savings in cooling costs. Just as important, they can sustain the large current density needed for many superconductor-magnet applications currently proposed.
Consider, for example, the ceramic HTS material yttrium-barium-copper-oxide (YBa.sub.2 Cu.sub.3 O.sub.7), sometimes referred to as a "1-2-3" superconductor. It has an anisotropic crystalline structure characterized by sheets formed of copper and oxygen in parallel a-b planes such that electric current flows best when channeled along the a-b planes and less easily in a perpendicular direction (i.e., the c-axis). Along the a-b planes, 1-2-3 can sustain a current density over 10,000 amperes per square centimeter (10.sup.4 A/cm.sup.2) at 77 kelvins in a one-tesla magnetic flux density. Such high current capability makes it a good candidate for various superconducting magnet applications.
Some efforts at adapting 1-2-3 to the task focus on forming it into wire (or tape). Doing so seems worthwhile because existing superconducting magnets cooled with liquid helium use wire. But ceramic HTS wire is brittle, and brittleness severely complicates the process of making a coil from fine HTS strands that can withstand the forces encountered. Moreover, making a ceramic HTS material sufficiently ductile and malleable to be formed into and used as wire can impair its current-carrying ability.
Trapping a magnetic field in a shell or cylinder of superconducting material would avoid wire and tape fabrication, but that approach involves certain other problems. To see why, recall that existing fabrication methods for trapping a magnetic field in a block of superconductive material proceed by using an electromagnet to apply a magnetic field to the material while the material is at a temperature above its critical temperature, T.sub.c, (i.e., that at which it becomes superconducting). Next, the temperature of the material is lowered to less than T.sub.c. Then, the electromagnet is de-energized and that results in currents flowing within the superconductor which at least partially maintain the applied magnetic field.
Those steps trap the magnetic field, but for HTS materials they can require an applied magnetic field having a magnetic flux density about three times as large as the desired flux density of the magnetic field to be trapped. So the process involves some difficulty and expense, especially to trap a magnetic flux density as large as one or two tesla.